Synthetic textile yarns such as nylon and polyester, and also rayon yarns, are conveniently spun as a bundle or tow of endless filaments, and cutting the tow into short lengths called staple fibers is a difficult problem, particularly when high rates of production are required, and also where it is necessary to cut the tow into very short staple lengths.
Many types of devices have been devised for cutting a filamentary tow bundle into short lengths. Early conventional cutting processes involved moving the cutting edge slightly lengthwise of the bundle as the cut was made because the bundle was moving lengthwise past the cutting edge during the cut. This sliding movement caused friction which tended to dull the cutting knives and make frequent knife replacement necessary. In addition, friction causes the generation of undesirable heat which, in instances, fuses the ends of synthetic filaments together. Also, the cut surface of the filaments would not be perpendicular to the longitudinal axis, but would be at an angle, because of the continuous longitudinal movement of the filament with respect to the cutting edge of the knife.
Other disadvantages of these early cutters relate to undesired changes in the properties of the material being cut. For instance, the motion of the cutting blades relative to the tow tends to cause undrawn (not crystalline oriented) filaments to be stretched unevenly during cutting. Additionally, the difficulty in correlating tow feed speed and cutter blade movement causes a range of lengths to be present in the cut staple--an undesirable property in a number of applications.
One attempt to overcome the disadvantages of prior art cutters is illustrated in U.S. Pat. No. 3,485,120. In this device, knives are mounted around a wheel with the blades extending radially outward. The tow is wrapped under light tension around the wheel and pressure is applied to cut the tow by means of a roller pressing against the outer surface of the tow wrapping as the wheel rotates. However, there are some disadvantages to this arrangement, as the distance between cutting blades is greatest where the cut occurs, and the cut staple does not tend to emerge from between the blades freely for short cut lengths. Also, centrifugal force tends to urge the movement of the cut staple toward the cutting edges, and the staple must be expelled in the opposite direction toward the center of the wheel.
Another apparatus using a somewhat different principle is illustrated in U.S. Pat. No. 3,768,355 which uses a power operated rotatable hollow shaft providing a passageway for the tow, the configuration of the shaft forcing the tow against knives which are arranged radially with their cutting edges pointing inward. This arrangement of knives overcomes some of the disadvantages of the design where the cutting edges pointed outward. However, a disadvantage is that the tow slides against a metal surface, thus tending to generate heat by friction, which might cause fusing and/or distortion of filaments at higher speeds. The metal surface is subject to rapid wear and relatively expensive hardened metals must be used in parts of the cutter subject to yarn-to-metal friction. Additionally, the cutting blades are stationary and there is no centrifugal force tending to move the cut staple away from the blades.
Still another apparatus is disclosed in the German Utility Model No. 7,331,413. With this apparatus, the yarn bundle to be cut is fed, while under tension, to a ring of blades and a roller cage positioned concentric therein, the ring of blades and the cage being rotatable relative to each other, and an endless belt is provided on the rolls so that the rolls do not come into direct contact with the yarn bundle.
The construction of this machine is fairly complicated and the apparatus is not very suitable for operation at high speeds.